Method of photographic printing of color records



Jan. 18, 1966 D, M. NEALE ETAL 3,229,574

METHOD OF PHOTOGRAPH 1C PRINTING OF COLOR RECORDS Filed March 51, 1965 4Sheets-Sheet 1 4- Lu I INVENTORfi Jaws Mwvzraow/lwzs 191552715X 6-".ZW/v/v/M Av'e 'y M Jan. 18, 1966 D. M. NEALE ETAL 3,229,574

METHOD OF PHOTOGRAPHIC PRINTING OF COLOR RECORDS Filed March 51, 1965 4Sheets-Sheet 2 Jan. 18, 1966 I D. M. NEALE ETAL 3,229,574

METHOD OF PHOTOGRAPHIC PRINTING OF COLOR RECORDS Filed March 31, 1965 4Sheets-Sheet 5 IN VENTORS fl-wxs Mxm raawA Zww 41550-5? M/V/V/M Jan. 18,1966 D, M. NEALE ETAL 3,229,574

METHOD OF FHOTOGRAPHIC PRINTING OF COLOR RECORDS Filed March 31, 1965 4Sheets-Sheet 4 t I INVENTORfi Jaws Mflmrrzzow/lqzs United States Patent3,229,574 METHOD OF PHOTOGRAPHIC PRINTING 0F COLOR RECORDS Denis M.Neale and Albert H. Ninnim, both of Ili'ord,

Essex, England, assignors to Ilford Limited, Ilford, England, a Britishcompany Filed Mar. 31, 1965, Ser. No. 453,860 Claims priority,application Great Britain, Apr. 6, 1961, 12,467 61 5 Claims. (CI. 8824)This is a continuation-in-part of application Serial No. 181,615, filedon March 22, 1962, now abandoned.

The invention relates to a method and apparatus used in photographicprinting of colour records. In particular, it relates to photo-electricapparatus used to measure the light fluxes in three spectral bands astransmitted by a multicolour photographic record. The word measure ishere used to indicate that the light fluxes are assessed by comparisonwith a fixed light flux. The apparatus is thus clearly distinguishablefrom that class of apparatus in which a light flux is assessed bymeasuring the current delivered by a photo-electric cell, it beingassumed that the efliciency of conversion of light to electrical currentremains constant over a time amounting to minutes or even hours.

In the printing of photographic multicolour transparencies and colornegatives in particular, it is known practice to divert a proportion ofthe printing light transmitted by the negative and to cause it to fallon combinations of color-selective filters and photoelectric cells suchthat three electric currents are obtained proportional respectively tothe light fluxes transmitted by the negative in the red, green and bluebands of the spectrum of the printing light. The combinations of filterand photocell may all be operative simultaneously. In an alternativeknown type of apparatus, a single photocell is used in front of whichred, green and blue filters are moved in turn and in synchronism withthe insertion of red, green and blue filters in the path of printinglight reaching the photosensitive print material. In many known forms ofapparatus, the electrical current obtained from each of the threecombinations of filter and photocell is applied to apparatus forterminating exposure of the print material to light of the correspondingcolour, red, green or blue, as the integral of electrical currentagainst time reaches a selected value.

Known apparatus is prone to inconsistencies due to a variety of causes.One of the more elusive causes of inconsistency has now been identifiedwith variations of spectral sensitivity response of the photocell orphotocells with variation of temperature. The sensitivity, and inparticular the spectral sensitivity characteristic of a photoemissivecell, is dependent on very thin surface layers on the photocathode.Typically these surface layers contain caesium and this has a vapourpressure at room temperatures which is sufliciently high fordistillation to occur in the vacuum cell. As a result there is atendency for caesium to migrate from one part of the cell to another,the movement being always towards the coldest part of the photocell.Such migration will occur relatively rapidly if the cell is warmedrapidly, but to some extent it must be expected whenever the temperatureof the cell changes at all since it is not practicable to arrange thatall parts of the cell heat and cool at identical rates. When migrationof caesium occurs in this way, the sensitivity and spectral response ofthe photocell cathode are changed. The changes in spectral response leadto errors in assessment of the relative red, green and blue light fluxestransmitted by the colour negative. Known types of automatic printersuse photocel-ls in temperature controlled enclosures in an attempt toeliminate this source of instability. The method is not entirelysatisfactory, however, and other remedies have been sought.

"ice

One alternative approach has been described by Gundelfinger, Taylor andYancey (Journal of Photographic Science, vol. 8, No. 5 pp. 161170). Thiscomprises using each photocell-filter combination only as a nulldetector. Light transmitted through the negative is continuouslycompared with light from a stable reference source and the intensity ofprinting light is adjusted until the photocell indicates equality. Inthis way small variations in colour sensitivity of the photocell aremade unimportant. The method described by Gundelfinger et al. involvesadjusting the light transmitted by the negative to a substantiallyconstant intensity and quality so that all prints are made with equalexposure times. In such a system, it follows that the printing exposuretime is determined by the amount of light transmitted by a negative ofmaximum optical density when all the available printing light is used.Even negatives of low optical density will require the same printingtime. In consequence the average exposure time is much longer than couldbe obtained with a similar light source in the more commonly used typeof printer which uses all the available printing light for printing'eachnegative and therefore prints each negative as quickly as possible.

The photoelectrical assessment method described by Gundelfinger et al.is not readily applicable to the more conventional types of printer justreferred to because when a variable light flux is transmitted by thecolour negative it is not readily compared with the reference lampcontinuously in a null manner. If such comparison is attempted, it isdiflicult to preserve the advantages offered by a photoelectricintegrating exposure time control, viz. that there is no need tostabilise the printing light source against the effects of supplyvoltage variations.

It is an object of the present invention to provide. as a step in theproduction of colour prints from photographic multioolourtransparencies, an improved method of assessing the intensity of lightin a spectral band.

According to the present invention there is provided a method for theproduction, from a colour transparency, of a multicoloured photographicprint on print material having components selectively sensitive to threespectral bands, by means of printing light containing light in each ofsaid three spectral bands, which method comprises applying in respect ofeach spectral band separately the procedure of allowing light from astable reference source to fall on light sensitive means, such as aphotocell, after passing through a colour-selective filter chosen sothat the combination of colour filter and photocell produces a responseconfined to the band of spectral sensitivity of only one component ofthe photographic print material, adjusting the sensitivity of a circuitassociated with the photocell so that the electrical output from saidcircuit assumes a preselected value, maintaining the said adjustedsensitivity constant while light from said reference source is removedor obstructed, directing printing light on to said photocell through acolour selective filter selecting substantially the same spectral band,and assessening the exposure of the print material to printing light insaid spectral band in accordance with the resultant output of saidelectrical circuit.

According to a preferred form of the present invention, the photocellincludes a secondary emission electron multiplier, the currentamplification factor of said electron multiplier being controlled byadjustment of voltage applied to saidelectron multiplier, saidadjustment being provided by an electrical power amplifier responsive tochanges in said electrical output so long as the said photocell receiveslight from said stable reference source.

Since photographic colour printing machines have already been used forseveral years in which some of the elements of the present inventionalso appear, it will now be shown in what fundamental manner the presentinvention differs from such earlier practice.

To this end reference is made to the accompanying drawings in which:

FIG. 1 illustrates an electronic circuit used in a first embodiment ofthe present invention.

FIG. 2 illustrates a printing machine of known type which may be used inconjunction with the circuit of FIG. 1.

FIG. 3 illustrates a printing machine according to a preferredembodiment of the invention.

FIG. 4 illustrates a motor and switch assembly used in said firstembodiment.

FIG. 5 illustrates an electronic circuit used in said preferredembodiment.

Referring to these drawings, FIG. 2 represents diagrammatically theprincipal optical components of a colour printing machine according tosaid earlier practice. In FIG. 2 light from a lamp 1 passes through acondenser lens 2 and a colour selective filter sector 3 of a tricoloursector disc 4 to strike a mirror 5 whence it is reflected to strike afield lens 6 and the multicolour negative 7. The lens 8 receives some ofthe light transmitted by the negative and forms an image of the negativein the plane of the print material 9. A partially reflecting plate 10diverts parts of the light passing the lens 8 to pass a further lens 11and a colour selective filter sector 12 of a tricolour sector disc 13and so fall on a multiplier photocell 14. The filter sectors 3 and 12 ofthe discs 4 and 13 are of like colour. Moreover the discs are coupled sothat they may be rotated to bring into operation other filter sectors3', 12 and 3 and 12". The three sectors on each disc transmit red, greenand blue light respectively and the sectors in operative position on thetwo discs are always of like colour.

When a print is not being exposed, a shutter 15 obstructs light passingthe lens 8 and a reflector 18 moves to direct on to the multiplierphotocell 14 light from a stable reference lamp 19. The sensitivity of amultiplier photocell depends critically on the voltage applied to theelectron multiplier. Commonly the individual secondary emissionelectrodes, known as dynodes, are connected to tappings on a resistancechain connected across a voltage supply hereinafter referred to as thedynode supply. It will be understood by those skilled in the art that itis readily arranged that the dynode supply voltage shall be adjustedautomatically so that, irrespective of substantial changes inphotocathode sensitivity or secondary emission efiiciency of thedynodes, the output current from the multiplier photocell collectorshall be held closely to a preselected value.

When a print is to be exposed, the dynode supply voltage is initially atthe automatically determined value. The automatic control is thendisabled so that the dynode supply Voltage remains constant at theautomatically determined value and is held at this value irrespective ofchanges in photocell collector current occurring during the print cycle.The reflector 18 then moves so that light from the reference lamp 19 nolonger strikes the photocell 14. Next, shutter 15 moves to allow lightpassing the negative '7 to fall on print material 9 and photocell 14 ashereinbefore described. Printing now commences, the print material 9being exposed to light of the colour transmitted by the filter sector 3.The filter sector 12 is intended to modify the spectral response of thephotocell 14 to match closely the spectral response of that emulsionlayer of the print material 9 which is sensitive to light of the colourtransmitted by the filter sector 3. The collector current drawn by thephotocell 14 in this condition is proportional to the integrated lightflux transmitted by both the filter sector 3 and negative 7. The saidcollector current is used, according to means familiar to those skilledin the art, to terminate the printing exposure by causing shutter 15 tomove into the closed position when a preselected integral of collectorcurrent against time has been drawn by the photocell 14. When thishappens, filter sector discs 4 and 13 each rotate to bring colouredsectors 3' and 12 into operative position. The shutter 15 then reopensand again closes when a second preselected integral of collector currentagainst time has been drawn by the photocell. The discs 4 and 13 thenrotate to bring sectors 3 and 12" into operative position and a finalexposure is given to the print material. It will be understood that thethree exposures in a complete print cycle comprise exposure to the red,green and blue bands of the spectrum separately. The component colour exposures may usually be administered in any convenient order provideddics 4 and 13 move in step with each other and in correct relation tothe circuits determining the preselected value of each collector currentintegral. At the completion of the print cycle, shutter 15 obstructslight passing lens 8, reflector 18 moves to direct light from lamp 19 onto photocell 14 and the automatic control of dynode supply voltagebecomes operative once more.

Photographic colour printing machines of the type just described sufferfrom the aforementioned effects of temperature changes on the spectralsensitivity of the photocathode of the photocell 14. Consequently, theexposure integrals of the red, green and blue components do not remainin the required proportion although, by virtue of the automaticsensitivity adjustment, the total exposure administered to the printmaterial remains substantially constant. After processing, prints aretherefore apt to show irregularity of colouring, although their meandensity is usually well controlled. Some improvement in colour stabilityis obtained by enclosing the photocell 14 in a temperature controlledhousing. In practice, how ever, this is found to be only a partialsolution.

In the known practice described above, light from the stable referencelamp 19 reaches photocell 14 only through filter 12. If, for example,filter 12 is a green filter, the automatic control of dynode supplyvoltage will correct for any variations in sensivity of photocell 14 togreen light. Accordingly, when the print cycle is initiated, the correctexposure of print material to green light is administered. If, however,the sensity to red light or to blue light has not changed in proportionto the sensitivity to green light, the exposure of print material to redlight and to blue light will be incorrect and the print will show acolour cast after processing to colour.

In the present invention, this limitation is overcome by arranging that,immediately before the multiplier photocell is used to assess theintensity of printing light of any of the three colours, red, green andblue, the photocell is exposed to light from a stable reference lampwhich light has passed through a filter corresponding to the colour oflight to be assessed in the immediately following operation and thedynode supply voltage is adjusted to provide a predetermined output fromthe multiplier photocell while so exposed to the stable reference lamp.

To facilitate comparison with the known practice described above, anembodiment of the invention will now be described with reference to theprinting machine of known type shown in FIG. 2. All the parts shown inFIG. 2 are still used, but the cycle of operations is different.

In the embodiment shown in FIG. 2, the standby condition corresponds toshutter 15 closed and light from reference lamp '19 reflected byreflector 18 to pass through filter sector 12 and fall on photocell 14.The automatic control of dynode supply voltage thus operates to set thesensitivity of photocell 14 so that the emission of light from lamp 19in the 'band passed by filter 12 produces a preselected collectorcurrent from the multiplier photocell. At the start of the print cycle,the sensitivity of the photocell 14 is thus already set to controlaccurately exposure of the print material to light transmitted by filtersector 3 and negative 7. On initiation of the print cycle, therefore,the photocell dynode supply voltage is disabled, reflector 18 moved toremove reference lamp illumination of the photocell and shutter 15opens. At the attainment of a preselected integral of collector currentagainst time, the shutter 15 closes and discs 4 and 13 rotate to bringsectors 3 and 12 into operative position.

At this stage in the print cycle, the method of the present invention isclearly seen to differ from that of the known practice described above.After sectors 3' and 12' have moved into operative position, thereflector 18 once more moves into position to reflect light from lamp 19on to photocell 14. The automatic control of dynode voltage is againmade operative so that a preselected collector current is obtained fromthe photocell when illuminated by that spectral band of the emission oflamp '19 transmitted by filter sector 12'. When sulficient time has beenallowed for the automatic control of dynode supply voltage to becomefully effective with filter 12' operative, the dynode supply voltage isagain disabled, the reflector 18 again removed and shutter 15 opens toinitiate printing with light transmitted by filter sector 3 and negative7. The photocell collector current is again integrated until apreselected integral of current again-st time is reached, whereuponshutter 15 closes and discs 4 and 13 rotate to bring sectors 3" and 12"into operative position. Reflector 18 again moves to reflect light fromlamp 19 through sector.12" on to the photocell 14. The automatic controlof dynode voltage is again made operative so that a preselectedcollector current is obtained from the photocell when illuminated bythat spectral band of the emission of lamp 19 which is transmitted byfilter sector 12'. When suflicient time has again been allowed for theautomatic control of dynode supply voltage to become fully effectivewith filter 12" operative, the dynode supply. voltage is again disabled,the reflector 18 again removed and shutter 15 opens to initiate printingwith light transmitted by sector 3" and negative '7. The collector,current is again integrated and when a preselected integral of currentagainst time has been reached, shutter 15 closes to terminate the entireprint cycle. Discs 4 and 13 now advance to bring sectors 3 and 12 oncemore into operative position, reflector 18 moves into operative positionand the automatic control of dynode supply voltage becomes operative sothat the apparatus is prepared for the initiation of another printcycle.

' It will be understood that, whereas the known earlier practice citedinvolved a single standardisation of photo- 'cell sensitivity against areference illumination containing only one of the three spectral bandswhich were to be separately assessed photoelectrically, the presentinvention proposes the standardisation of photocell sensitivity againstreference illumination confined to the spectral band, typically red,green or blue, which is to be assessed T-by that photocell in theimmediately following operation. .Thus, whereas the earlier knownpractice employed only a single standardisation operation before thephotocell asisessed three different spectral bands, the presentinvention utilizes a separate standardisation of photocell sensitivity.with respect to each separate spectral band to be as- .sessed. 1

Reference will now be made to FIGS. 1 and 4 which represent electricalcircuits and a motor arrangement contacts of relays A/ 4 and B/4 havebeen drawn in positionscorrespond-ing to the d e-energized condition ofrelays A/4 and B/4. When the printer is in a stand-by "condition, i.e.when it is not printing, relay A/4 is de- -ener'gized, relay B/4 isenergized. Accordingly, during standby, the collector 2-2 of photocell14 is connected by contacts b/1 to a load resistor R2.

Light from the reference source 19 falls on the photocathode 23 ofphotocell 14 and gives rise to a current which is drawn by collector 22through R2 from supply E2. Cathode follower V2 applies to the controlgrid of series valve V1 a potential about two volts in excess of thecollector potential of photocell 14. If the collector current is greaterthan is required, the voltage drop across R2 will be abnormally largeand the grid of V1 will accordingly be held less positive than normal.The cathode of V1 is, however, held at a positive potential by the glowstabilizer V6. Consequently an excessive collector current fromphotocell 14 leads to an increased grid bias on V1. As a result, V1passes a reduced current through V6 and R1 back to the floating voltagesupply, E1. The reduced current through R1 produces a reduced dynodesupply voltage across R1 and as a result the amplification factor of theelectron multiplier is reduced. This in turn causes the collectorcurrent of photocell 14 to diminish. A state of equilibrium is reachedwhen the dynode supply voltage leads to a collector current providingthe bias on V1 required to maintain said dynode supply voltage. Theoperating voltage of the glow stabiliser tube V6 is chosen to be greaterthan the bias voltage which must be applied between grid and cathode ofV1 substantially to cut off anode current in V1. The voltage of supplyE2 is made several times the operating voltage of V6. It followstherefore that although the sensitivity of photocathode 23 may varyconsiderably, the dynode supply voltage will be adjusted automaticallyto reduce the variations in collector current to a much smallerpercentage.

In FIG. 4 a motor 24 is shown coupled to a shaft 25 to which are fixedcams 26, 27, 28 and the driving disc 29 of a three-station Genevamotion. For each complete rotation or shaft 25, pin 30 engages one ofthe slots 31, 31', 31", in the star wheel 32, turning shaft 33 throughof rotation. Thereafter pin 30 disengages said slot and shaft 33 islocked against further rotation because locking plate 34, attached todisc 29, prevents star wheel 32 from turning. Shaft 33 is coupled todisc 13 (FIG. 2) so that for each revolution of shaft 25, one of thefilters 12, 12, 12" is removed from operative position and the nextfilter is brought into such position.

In FIG. 4, the cams and Geneva motion are in the stand-by condition,i.e. with the machine not printing. Contacts b/4 are shown in theposition corresponding to relay B/4 being de-energized. As shown in FIG.4, motor 24 is de-energized because earns 35 and 28 hold contacts m/1and m/Z open. Carn 27 holds contact m/ 3 open and so relay A/4 (FIG. 1)is de-energized, leaving mirror 18 in position to direct light fromsource 19 through filter sector 12 to photocell 14. Relay B/4 isenergized by current flowing through hold-on contacts b/3 and a/4, relayB/4 having been energized during a preceding cycle of printing. Contactsb/2 are therefore closed, holding solenoid L2 energized and thus holdingshutter 15 in position to'obstruct the path of light passing lens 8.Contacts b/l are in position to connect collector 22 to load resistor R2so that the dynode supply voltage is adjusted automatically, asdescribed above, to produce a predetermined collector current when lightfrom source 19 passes filter 12 and falls on photocathode 23.

To initiate a printing cycle, key switch 36 is closed momentarily.Current then flows from mains supply 37, through key switch 36 to themotor and returns through contacts m/4 to supply 37. As shaft 25 turns,contacts m/2 close so that even when key switch 36 is opened, motor 24continues to drive shaft 25. Cam 27 now allows contacts m/3 to close,energizing relay A/4, opening contacts a/l and disabling the automaticcontrol of dynode supply voltage. Capacitor C1 now communicates to thecontrol grid of V2 any change in dynode supply voltage due, for example,to variation of supply voltage E1. Any such change is conveyed to thecontrol grid of V1 by the cathode follower V2 and the resultant changein conductance of V1 serves to oppose the change in dynode supplyvoltage. Once contacts a/1 open, therefore the dynode supply voltage ismaintained at the value established immediately prior to opening ofcontacts a/ 1.

Energization of relay A/4 closes contacts a/2 energiz ing solenoid L1which withdraws mirror 18 from operative position. Contacts a/4 open, soallowing relay B/4 to deenergize. Contacts b/1 therefore return to theposition shown in FIG. 1 and contacts a/3 open so that current drawn bycollector 22 charges capacitor C2.

V3 and V4 comprise an operational amplifier of known type. The feedbackcapacitor C2 serves as a current integrator. Current drawn by collector22 produces only a small depression of the voltage at the grid of V3 buta much larger rise in potential of the cathode of V4.

De-energization of relay B/4 causes contacts [2/2 to open, de-energizingsolenoid L2 and allowing shutter 15 to be withdrawn by a return spring39 from the path of printing light. De-energization of relay B/4 alsocauses contacts 11/4 to open so that motor 24 is de-energized when cam26 opens contacts m/4. When this happens, disc 29 stops with pin 30 inthe position shown dotted at 30.

Printing of the negative 7 now proceeds with light passing filter 3.Part reflector 10 directs a proportion of printing light through filter12 to photocell 14. C01- lector current from photocell 14 is integratedby capacitor C2, causing the cathode potential of V4 to rise untiltrigger tube V conducts energizing relay B/ 4.

Energization of B/ 4 closes contacts b/2 energizing solenoid L2 andcausing shutter 15 to obstruct the path of printing light. Contacts b/lre-connect collector 22 to load R2. Contacts b/4 close to energize motor24, so causing pin 30 to turn the star wheel 32 and filter discs 13 and4 through 120 of rotation. Cam 27 opens contacts m/3 so that relay A/4is de-energized. Contacts a/3 discharge capacitor C2, contacts a/2 opento cause L1 to allow the mirror 18 to move into operative position underthe action of a control spring 38. Contacts a/4 close, completing a holdon circuit for relay B/4 through contacts b/3 and R3. By making theresistance of R3 small, the voltage across R3 is arranged to be lessthan the voltage required to maintain a discharge from anode to cathodeof V5. Consequently the discharge in V5 is extinguished.

Contacts a/1 close when relay AM is de-energized. Consequently, theautomatic adjustment of dynode supply voltage becomes operative oncemore and adjusts the dynode supply voltage to a value giving apre-determined collector current in respect of light reaching photocell14 through filter 12' from reference source 19.

Motor 24 continues to drive shaft 25 until pin 30 once more reachesposition 30. The sequence already described is then repeated except thatprinting occurs with light passing filter 3 and assessment of printinglight is made by photocell 14 through filter 12' while the dynode supplyvoltage is maintained at a value established While light from source 19fell on photocell 14 through filter 12'.

When the integrated collector current has again charged capacitor C2sufliciently to cause trigger tube V5 to conduct, relay B/4 is againenergized and motor 24 turns shaft 25 through another completerevolution, as already described, filter 3' and 12' thereby beingremoved from operative position and filters 3" and 12" being broughtinto operative position. Before motor 24 comes to rest, cam 27 opens andcloses contacts m/ 3 so temporarily de-energizlng relay A/4 and causingthe dynode supply voltage to be adjusted as previously described to avalue producing the predetermined current at collector 22 when photocell14 receives light from source 19 through filter 12". When cam 27 allowscontacts m/ 3 to close again, another partexposure of the print material9 occurs, this time to light passing filter 3". Printing light isassessed by photocell 14 through filter 12" while the dynode supplyvoltage is maintained at the value established while photocell 14received ght from reference source 19 through filter 12".

When the integrated collector current has again charged capacitor C2sufiiciently to cause trigger tube V5 to conduct, relay B/4 is againenergized and motor 24 turns shaft 25 to rotate star wheel 32, filterdisc 4 and filter disc 13 through a further of rotation. This causes cam35 to open contacts m/1 so that when cam 28 opens contacts m/ 2 themotor is de-energized. The apparatus is thus left in the stand-bycondition, viz. with pin 30 in the position shown, with filters 3 and 12in operative position, with relay A/ 4 de-energized and relay B/ 4energized, with shutter 15 obstructing light passing lens 8 and withmirror 18 directing light from reference source 19 through filter 12onto photocell 14.

It will be understood that the embodiment of the invention as describedwith reference to FIG. 2 leads to a method of automatic photographicprinting which is slower and more complex than the known practicedescribed above on account of the two additional standardizationoperations which are required between the separately administeredexposures of the print material to light of colours transmitted byfilter sectors 3, 3 and 3". Ac-

cordingly, a preferred embodiment of the invention will be describedwith reference to FIG. 3 and FIG. 5.

In FIG. 3 the negative is initially illuminated by light containing allthree spectral bands, red, green and blue, required for printing thenegative 7 by projection through lens 8 onto the print material 9. Anyconvenient known type of lamphouse may be used, but shutters 45, 45'aiid 45" in FIG. 5 are required to terminate exposure to the red, greenand blue components of printing light separately.

Part of the light transmitted by the negative 7 strikes the reflectingsurface 21 and falls on three multiplier photocells 14, 14', 14" afterpassing through colour selective filters 12, 12', 12", respectively. Anopaque movable flap, shown at 21) in FIG. 3, can be swung to position20' wherein it obstructs light from the negative and surface 21 butallows light from reference lamp 19 to reach photocells 14, 14' and 14"after passing filters 12, 12', 12", respectively.

FIG. 5 represents an electronic circuit which may be used in conjunctionwith the embodiment of the invention shown in FIG. 3. FIG. 5 will beseen to comprise three similar sub-circuits. tocell 14 is closelysimilar to the circuit of FIG. 1, like parts being indicated by likereference letters and numbers. Sub-circuits associated with photocells14' and 14" are identical with that associated with photocell 14,corresponding parts being indicated by like reference letters andnumbers, parts relating to photocell 14 being identified by a singlestrike and parts relating to photocell 14" being identified by a doublestrike.

Before commencement of the print cycle, the movable flap is in position20 and each photocell is thus illuminated by a different spectral bandof the light emitted by reference lamp 19, said spectral bands beingselected by colour selective filters 12, 12', 12". Each multiplerphotocell is connected to a separate circuit providing automatic controlof its dynode voltage supply so that the collector current of eachphotocell assumes a preselected value when illuminated as described bythe lamp 19 through its associated filter 12, 12 or 12".

When key switch 40 is briefly closed to initiate a printing cycle, relayD/11 is energized, contacts d/4, d5, d/6 open and relays B/ 4, B'/ 4,B"/ 4 are deenergized. A holdon circuit for relay D/11 is thus formedthrough contacts d/7 and any of the contacts b/4, b'/4, b"/4. When keyswitch 40 is opened, therefore, relay D/11 remains energized.

Energization of relay D/11 opens contacts d/l, d/Z, d/3, therebydisabling the automatic control of dynode supply voltage to each of thephotocells 14, 14, 14". For each photocell the preadjusted sensitivityto the spectral band passed by its respective filter thereafter remainsconstant.

The sub-circuit associated with pho-- Solenoid L3, connected in parallelwith relay coil D/ 11 is also energized to cause the movable flap toassume the position shown at 20 in FIGS. 3 and 5. When in position 20,the flap obstructs light from lamp 1?, but allows light from thenegative 7 and reflector 21 to pass filters 12, 12', 12" to strikephotocells 14, 14, 14". Integration of each photocell collector currentnow begins separately. Energization of relay D/ 11 causes contacts d/8to close so that solenoid L4 is energized and shutter 15 opens to allowlight passing lens 8 to form an image of the negative 7 on printmaterial 9. As each photocell collector current produces a preselectedintegral of current against time, the corresponding trigger tube V V orV conducts to energize the corresponding relay B/4 B'/4 or B"/4.Contacts b/2, b/2 or b"/2 on said relay close to energize solenoid L2,LZ, or L2, so bringing into operative position the shutter 45, 45, or 45terminating exposure of the print material to the spectra-l bandcorresponding to the spectral transmission of filter 12, 12 or 12"associated with the photocell producing the said preselected integral ofcurrent against time.

When all three relays B/ 4, B/'4l, B"/4 have been energized, allcontacts b/4, b'/4, b"/4 are open, relay D/ 11 becomes de-energized,contacts d/8 open and solenoid L4 becomes de-energized also, allowingspring 42 to move shutter into position to obstruct the path of lightpassing lens 8. Solenoid L3 is de-energized also and spring 41 causesthe movable flap to return to the position shown at Relay contacts b/l,b/l, b"/1 return to that state which is the reverse of that shown inFIG. 5 and relay contacts d/ll, (l'/1, d"/1 close so that the automaticcontrol of dynode supply voltage to each photocell becomes operativeonce more.

It will be appreciated that the printing method described with referenceto FIG. 3 is considerably faster than that described with reference toFIG. 2. Moreover, the simplification of switching processes requiredduring a complete print cycle largely off-sets the complication involvedin providing three separate multiplier photocells with three independentcircuits for controlling automatically the dynode supply voltages.

The above-described embodiments are illustrative of the invention butare not intended to limit the possibilities of insuring that theintensity of light in a spectral band is accurately assessed. Theapparatus disclosed herein are examples of arrangements in which theinventive features of this disclosure may be utilized, and it willbecome apparent to one skilled in the art that certain modifications maybe made within the spirit of the invention as defined by the appendedclaims,

What is claimed is:

1. A method for the production, from a colour transparency, of amulticoloured photographic print on print material having componentsselectively sensitive to three spectral bands utilizing printing lightcontaining each of said three spectral bands, comprising for eachspectral band separately the steps of: directing light containing thethree-spectral bands from a stable reference lamp to a firstcolour-selective filter designed to pass light in the separate band ontolight sensitive means to generate a first electrical signal for saidband having a value corresponding to the light in the separate spectralband; adjusting said electrical signal from the light sensitive means inorder to bring said electrical signal to a preselected value, thereaftermaintaining the said adjusted sensitivity constant; interrupting thelight directed from said stable reference lamp; passing printing lightthrough the colour transparency onto said printing material to exposesame and passing printing light also through a filter transmitting lightin substantially the same spectral band as said colour-selective filteronto said light sensitive means to generate a second electrical signal;and interrupting the passage to the printing material and to the lightsensitive means of printing light in said separate band in response tothe value of said second signal as modified by a said adjustedsensitivity.

2. A method according to claim 1, wherein the printing light which isdirected to the said light sensitive means is such light after it haspassed through said colour transparency.

3. A method for producing a multicoloured photographic print as setforth in claim 1 wherein the procedure recited is consecutivelyperformed for each of the three separate spectral bands.

4. A method for producing a multicoloured photographic print as setforth in claim 1 wherein the procedure recited is simultaneouslyperformed for each of the three separate spectral bands.

5. A method for producing a multicoloured photographic print as setforth in claim 1 wherein said second electrical signal is integratedagainst time, the printing light being interrupted when the integratedsignal reaches a prescribed value.

References Cited by the Examiner UNITED STATES PATENTS 3,090,289 5/1963Gundelfinger 82-24 X NORTON ANSHER, Primary Examiner.

EVON C. BLUNK, Examiner.

R. A. WINTERCORN, Assistant Examiner.

1. A METHOD FOR THE PRODUCTION, FROM A COLOR TRANSPARENCY, OF AMULTICOLOURED PHOTOGRAPHIC PRINT ON PRINT MATERIAL HAVING COMPONENTSSELECTIVELY SENSITIVE TO THREE SPECTRAL BANDS UTILIZING PRINTING LIGHTCONTAINING EACH OF SAID THREE SPECTRAL BANDS, COMPRISING FOR EACHSPECTRAL BAND SEPARATELY THE STEPS OF: DIRECTING LIGHT CONTAINING THETHREE-SPECTRAL BANDS FROM A STABLE REFERENCE LAMP TO A FIRSTCOLOUR-SELECTIVE FILTER DESIGNED TO PASS LIGHT IN THE SEPARATE BAND ONTOLIGHT SENSITIVE MEANS TO GENERATE A FIRST ELECTRICAL SIGNAL FOR SAIDBAND HAVING A VALUE CORRESPONDING TO THE LIGHT IN THE SEPARATE SPECTRALBAND; ADJUSTING SAID ELECTRICAL SIGNAL FROM THE LIGHT SENSITIVE MEANS INORDER TO BRING SAID ELECTRICAL SIGNAL TO A PRESELECTED VALUE, THEREAFTERMAINTAINING THE SAID ADJUSTED SANSITIVITY CONSTANT; INTERRUPTING THELIGHT DIRECTED FROM SAID STABLE REFERENCE LAMP; PASSING PRINTING LIGHTTHROUGH THE COLOUR TRANSPARENCY ONTO SAID PRINTING MATERIAL TO EXPOSESAME AND PASSING PRINTING LIGHT ALSO THROUGH A FILTER TRANSMITTING LIGHTIN SUBSTANTIALLY THE SAME SPECTRAL BAND AS SAID COLOUR-SELECTIVE FILTERONTO SAID LIGHT SENSITIVE MEANS TO GENERATE A SECOND ELECTRICAL SIGNAL;AND INTERRUPTING THE PASSAGE TO THE PRINTING MATERIAL AND TO THE LIGHTSENSITIVE MEANS OF PRINTING MATERIAL AND TO THE LIGHT SENSITIVE TO THEVALUE OF SAID SECOND SIGNAL AS MODIFIED BY A SAID ADJUSTED SENSITIVITY.